Recently, as an image forming apparatus for outputting an image signal as a visible image onto a recording medium, such as paper, an image forming apparatus for directly forming a toner image on a recording medium by allowing toner, which is development particles, to fly so as to adhere to the recording medium is suggested (for example, Japanese Unexamined Patent Publication No. 6-155798/1994 (Tokukaihei 6-155798)). The following describes the above-mentioned conventional image forming apparatus in reference to FIGS. 15 and 16.
As shown in FIG. 15, the conventional image forming apparatus has an image forming section 51 having a toner supplying section 52, a printing section 53. The image forming apparatus allows toner 71 to fly so as to adhere onto a sheet 55 which is a recording medium. At the same time, the image forming apparatus controls the flying of the toner 71 based upon an image signal, an image is directly formed on the sheet 55.
The toner supplying section 52 is composed of a toner storing tank 70 where the toner 71 as negatively charged development particles is stored, and a toner holder 72 for holding the toner 71 by magnetic force. The toner holder 72 is grounded, and it rotates in the direction of the arrow "E" shown in FIG. 15 so that its speed on the surface is kept 30 mm/sec. The toner 71 is magnetic toner with average particle diameter of 10 .mu.m, and it is charged by known technology so that the charging amount is -4 .mu.C/g to -5 .mu.C/g. Moreover, the toner 71 is held on the outer peripheral surface of the toner holder 72 so that a layer with an average thickness of 80 .mu.m is formed.
The printing section 53 of the image forming section 51 has a counter electrode 75 composed of an aluminum tube with a diameter of 50 mm, and a control electrode 76 provided between the counter electrode 75 and the toner holder 72. The counter electrode 75 is provided so that the distance from the outer peripheral surface of the toner holder 72 becomes 1 mm. A voltage of 2 kV is applied to the counter electrode 75 by a high voltage power source 80, and it rotates in the direction of the arrow "F" shown in FIG. 15 so that its speed on the surface is 30 mm/sec. In other words, an electric field, which is necessary to allow the toner 71 held on the toner holder 72 to fly towards the counter electrode 75, is formed between the counter electrode 75 and the toner holder 72.
The control electrode 76 is parallel with a tangent plane of the outer peripheral surface of the counter electrode 75 and two-dimensionally spreads opposite to the counter electrode 75 so that the toner 71, which flies from the toner holder 72 towards the counter electrode 75, can pass the control electrode 76. The electric field formed between the toner holder 72 and the counter electrode 75 is changed by a potential supplied to the control electrode 76, and the flying of the toner 71 from the toner holder 72 to the counter electrode 75 is controlled.
The control electrode 76 is provided such that the distance from the outer peripheral surface of the toner holder 72 becomes 100 .mu.m. The control electrode 76 is composed of a flexible print substrate (FPC) 76a with a thickness of 50 .mu.m and ring electrodes 77 made of copper foil with a thickness of 20 .mu.m. Gates 79 with diameter of 150 .mu.m which is a passing section of the toner 71 is formed on the substrate 76a, and the ring electrodes 77 are provided around each gate 79. Each ring electrode 77 is electrically connected to a control power source section 81 via a feed line and a high voltage driver (not shown).
A voltage according to an image signal is applied to the ring electrodes 77 by the control power source section 81. In other words, in the case where the toner 71 held on the toner holder 72 is allowed to pass towards the counter electrode 75, the control power source section 81 applies a voltage of 200 V to the ring electrodes 77. Meanwhile, in the case where the toner 71 held on the toner holder 72 is not allowed to pass towards the counter electrode 75, the control power source section 81 applies a voltage of -200 V to the ring electrodes 77. In such a manner, when the applying voltage to the control electrode 76 is controlled according to an image signal and a sheet 55 is positioned on the counter surface on the counter electrode 75 to the toner holder 72, a toner image according to the image signal is directly formed on the surface of the sheet 55.
The following describes the electric field, which is formed between the toner holder 72 and the counter electrode 75 by applying voltage thereto. As mentioned above, since the toner holder 72 is grounded and a voltage of 2 kV is applied to the counter electrode 75, an equipotential surface in the range from 0 V to 2 kV is formed between the toner holder 72 and the counter electrode 75 at equal intervals. Since the counter electrode 75 is provided so that the distance from the outer peripheral surface of the toner holder 72 becomes 1 mm and the control electrode 76 is provided so that the distance from the outer peripheral surface of the toner holder 72 becomes 100 .mu.m, a potential (Vc) of the center portion of the gates 79 on the control electrode 76 is 200 V.
In this state, a voltage (Ve) of 200 V is applied to the ring electrodes 77 on the control electrode 76 by the control power source section 81 so that the toner 71 held on the toner holder 72 is allowed to pass towards the counter electrode 75. Namely, the voltage is applied to the ring electrodes 77 so that Vc=Ve. Then, as shown in FIG. 16, the toner 71 flies in the perpendicular direction to the equipotential surface by a potential difference between the toner holder 72 and the counter electrode 75, and reaches the surface of the counter electrode 75, namely, the sheet 55. As a result, a dot having the same diameter as of the gates 79 (150 .mu.m) is formed on the sheet 55. In other words, a dot diameter (FL) of an image (not shown) formed on the sheet 55 becomes equal to the diameter (dm) of the gates 79 (dm=FL).
The toner 71, which flew from a portion on the surface of the toner holder 72 which does not corresponds to the gates 79, cannot pass the gates 79, and adheres to the control electrode 76. In other words, the toner 71 which can pass the gates 79 is limited only to the toner 71 which flies from the portion corresponding to the gates 79 on the toner holder 72.
In above-mentioned conventional image forming apparatus, a diameter of a dot formed on the sheet 55 is equal to of the gates 79. In other words, resolution of an image formed on the sheet 55 is determined by the diameter of the gates 79. Consequently, in order to improve the resolution of an image in a conventional image forming apparatus, the diameter of the gates 79 on the control electrode 76 should be further minimized.
However, in order to further minimize the diameter of the gates 79 while reliability of an apparatus is maintained, namely, in order to manufacture the control electrode 76 so that the gates 79 having a smaller diameter are formed, a high-precision manufacturing technique is required. Accordingly, since yield of the control electrode 76 is lowered, cost rises. Therefore, in a conventional image forming apparatus, it is difficult to improve resolution of an image without a rise of its cost.
In order to minimize a diameter of a dot formed on sheet 55, it is considered that the distance between the control electrode 76 and the counter electrode 75 or a voltage to be applied to the counter electrode 75 is changed. However, if the distance or the voltage is changed, the state of the toner 71 flying towards sheet 55 is degraded, thereby causing a new problem of a deterioration in image quality, such as a distortion of a shape of a dot.
In addition, if a voltage is applied to the ring electrodes 77 so that Vc&lt;Ve, after passing the gates 79, the toner 71 receives electric force due to an electric field adjacent to the gates 79, and the toner 71 spreads wider than the gates 79. Moreover, Coulomb's force (repulsive force), which is generated by charges of the toner 71, further widen the spread of the flying toner 71. Consequently, the toner 71 scatters and a diameter of a dot formed on sheet 55 becomes large. Accordingly, since toner density of a dot becomes low, sufficient toner density to form an image with good quality cannot be obtained. For this reason, a contrast becomes poor and contour becomes indistinct, so resolution of an image deteriorates.
In order to obtain sufficient toner density, it is necessary to make a toner layer (an amount of toner 71 held on the outer peripheral surface of the toner holder 72) thicker or to further increase the revolution rate of the toner holder 72. However, if the toner layer is made thicker, the uniformity of the toner layer and the uniformity and the stability of a charging amount of toner 71 are deteriorated. Moreover, if the revolution rate of the toner holder 72 is increased, vibration which is generated from the toner holder 72, a driving unit for driving the toner holder 72, etc. becomes large accordingly, thereby causing a new problem that the reliability of the apparatus is declined.
In addition, in order to obtain sufficient toner density, it is considered that time for continuously giving a potential, which allows the toner 71 to pass, to the control electrode 76 is made longer and to increase a flying amount of the toner 71. However, in this case, a time for forming an image becomes long. Consequently, an image forming speed is slowed down, and the toner 71 is remarkably scattered, thereby further deteriorating quality of an image.
In addition, in order to obtain resolution of 300 DPI (dot per inch), for example, it is necessary to adjust each dot diameter to approximately 150 .mu.m. However, in a conventional image forming apparatus, in order to adjust each dot diameter to the above value, a diameter of gates 79 on control electrode 76 should be made smaller than at least the dot diameter, namely, 150 .mu.m. Therefore, a high-precision manufacturing technique is required for manufacturing control electrode 76 in this manner, and accordingly, since yield of control electrode 76 is lowered, cost rises. Moreover, since a diameter of gates 79 becomes too small, the gates 79 become clogged with toner 71, thereby making it impossible to stably form an image with good quality on a sheet 55.
Since it is considered that a demand for the improvement of resolution is further increased from now on, if the above problem is solved only by improving accuracy, more difficulties are expected.